More devices are preferably, or now essentially, wirelessly powered and this trend is expected to continue with the increasing global demand for and acceptance of such devices. Ideally, such wireless power transfer must take place at very high frequency to enable compact and elegant designs. However, the current wireless power technology is still at a stage where it can support ‘efficient’ wireless power transfer only at sub MHz or well below the desired frequencies.
Typically inductive power transfer (IPT) systems are operated at relatively low frequencies in the range of 10 kHz to 200 kHz to avoid unacceptable levels of switching losses, incurring at higher frequencies. However, there has always been a demand for efficient operation of IPT systems at much higher frequencies, in the range of several MHz, to allow for the design of compact and efficient consumer electronics. Existing IPT systems are not suitable for operation in the MHz frequencies being inefficient due to unavoidable high losses at elevated frequencies. This can be caused by switching losses incurred for each switching operation, as well as the inefficiency of monitoring the zero voltage and/or current switching timings.
A push-pull resonant inverter as described by Hu et al (US 20080247210) involves an input power source which is divided between two inductive elements. Operation of the switches enables an appropriate current to flow in the resonant circuit and generate fields suitable for wireless power transfer. The push-pull resonant converter is typically loosely coupled to one or more passive, rectifying pick-ups with a connected load. Although the push-pull resonators are popular, controlling the switching means may be complicated, especially as the frequency of operation increases. In particular the switching frequency is often load dependent and must be determined by monitoring the circuit for zero voltage or current situations.